Is there a way (either an existing property or a mechanism for providing such a property) for my build.xml to be aware of which user requested a personal build?

My team would like to create outputs for personal builds in folders identified by the userID of the build requester. (These folders/outputs are created on the build engine and are web accessible.)

I know that entries on the Build Definition's properties tab are available to ant, but I don't know how to populate such a field. (Even better would be an existing property that I can simply reference.)

Note: Ideally there'd be no white space in the name (though of course I can process it if an ID containing white space is all that's available). So if one property has my name "Brian Murray" and another my ID "shonuff", I'd prefer to use the latter.